This invention relates to turbine/compressor air cycle systems and more particularly to such an air cycle system wherein a single shaft air cycle machine provides alternatively, both a simple/recirculation cycle and a simple/bootstrap cycle.
It is a known art to condition air for the cabin of an aircraft by causing high temperature bleed air to work in a turbine to lower its temperature and pressure preparatory to ingress to the cabin. Bleed air is typically supplied by the supercharger of a piston engine, by an auxiliary power unit or engine (APU) or from a gas turbine engine. At various times, the bleed air is at various pressures. It may be, for example, that while the aircraft is at cruise power or higher or is on the ground with bleed air being supplied by a high pressure APU, the bleed air is at relatively high pressure (e.g., 50 psi). This is in contrast to when the aircraft engines are at ground idle or idle descent, in which case bleed air is at relatively low pressure (e.g., 25 psi). In the past, systems have been designed to maximize effectiveness of a high pressure bleed air supply by incorporating mechanisms which utilize a simple cycle, i.e., a system wherein relatively high pressure bleed air is fed directly to the turbine to do work therein and lower the temperature and pressure of the air. In the simple air cycle, the turbine uses the rotary power created by the high pressure air to drive a fan which pulls ram air or ambient air through a heat exchanger and then exhausts this air overboard. The bleed air is fed through the heat exchanger prior to entering the turbine so that the work performed by the turbine is also used to produce a cooling air flow in the heat exchanger to further cool the air from the bleed.
Prior art systems have also been designed to utilize low pressure bleed air by incorporating components wherein the bleed air is first fed to a compressor. The compressor, in turn, passes the air through the turbine to produce work in the well-known bootstrap air cycle. The term bootstrap relates to the fact that the compressor output is fed to the turbine which, of course, increases the power of the turbine and the turbine in turn drives the compressor.
In one unique example of a bootstrap-type air cycle system, reference is made to U.S. Pat. No. 3,428,242 to Rannenberg dated Feb. 18, 1969 and of common assignee as the present invention, the teachings of which are incorporated herein by reference. In that patent, a single shaft three-wheel machine includes, on a common shaft, a turbine, a compressor and a fan. The compressor receives the bleed air, passes it via a heat exchanger to the turbine for driving the turbine which, of course, drives the compressor. The turbine also drives the fan which functions to force ram or ambient air through the heat exchangers. Since the fan power is derived as in a simple cycle, and the compressor power as in a boot-strap cycle, this scheme has been referred to as a simple/bootstrap system. The air output of the turbine is the conditioned air for the cabin of the aircraft. One of the unique features of the device described by the aforesaid patentee is the fact that the conditioned air is passed over the bearings of the machine in order to cool them to temperatures compatible with shaft lubrication.
Although the prior art has recognized the usefulness of both high and low pressure bleed air and has provided means for utilizing one or the other, the need has remained for a single system which can utilize both in a simple and effective manner.